<p>The reuse of biomass waste in energy materials is of great significance for enhancing energy efficiency and achieving China’s goals of carbon peak and carbon neutrality. In this study, a novel approach is employed by initially preparing biomass porous carbon from corn stalk powders (hCC). Given its well-defined redox activity and higher theoretical capacitance, MnCO₃ is deposited on the surface of hCC using a hydrothermal method, resulting in the successful preparation of hCC@MnCO₃. Through electrochemical testing methods such as cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy, the electrochemical properties of hCC@MnCO₃ are also investigated in supercapacitors. The impact of varying hydrothermal durations (12, 24, and 36&#xa0;h) on the properties of hCC@MnCO₃ composites reveals that the hCC@MnCO₃-24 composite, obtained after a 24-h hydrothermal reaction, exhibits a high specific surface area (1351 m²&#xa0;g⁻¹) and a specific capacitance of 436.25 F g⁻¹ at a current density of 0.1 A g⁻¹. The galvanostatic charge–discharge performance of the hCC@MnCO₃-24 composite electrode in a symmetric device configuration demonstrates a high energy density of 12.15 Wh kg⁻¹, corresponding to a specific capacitance of 350 F g⁻¹ at 0.1 A g⁻¹. The hCC@MnCO₃-24 electrode exhibits exceptional cycling stability, Maintaining a capacitance retention rate of 94.44% after 10,000 charge–discharge cycles at a current density of 0.4 A g⁻¹.</p>